Would you rebush these con rods and how do you chamfer?

[Mike Juzenas]

Rebuilding an engine is like building a house...a million little decisions...

I have measured the rods that I have taken out of my 96 993 (well, I was doing a valve job and then decided to go all the way...in retrospec I did not need to do it but now I know exactly what I have) and they measure out like:

Up-down clearances between the rod and wrist pin (spec is 0.020-0.037)

Rod 1 - 0.030
Rod 2 - 0.028
Rod 3 - 0.028
Rod 4 - 0.028
Rod 5 - 0.028
Rod 6 - 0.028

Side to side clearance between the rod and wrist pin (spec is 0.020-0.037)

Rod 1 - 0.020
Rod 2 - 0.020
Rod 3 - 0.020
Rod 4 - 0.020
Rod 5 - 0.020
Rod 6 - 0.020

So this means that the side to side clearances (which logically have very little wear) are exactly the minimum that Porsche specs which means practically zero wear.

The up-down measurement shows that the bushing are very slightly oval but still inside spec.

Okay, to make them perfectly round again I need to have new Porsche bushings pushed in again (check..I have those) and then have them reamed for 0.020 clearance (check...the machine shop says no problem).

So why am I hesitating? Well, two issues:

1 - There is a small risk of changing a factory installed bushing, however small.

2 - My machine shop says they cannot chamfer the holes on the inside (see photo). Would this cause problems?

If I know how to chamfer the hole then it marginally makes sense to redo the bushings...more of a all-new-bearings syndrome than actual practical advice. and I like the idea of the bearings being totally round again.

The large end bushing will be done anyways, I am changing over the ARP bolts and new bearings..

Cheers,

Mike

[davidppp]

Hello, Mike.

I don not know anyone who measures these..

Do they rock on the pins?
None is acceptable..

Kind regards
David

[Mike Juzenas]

Very slight rock, definitely not tight. The rock is more pronounced up and down but not to the side...its much more suttle. But the spec says 0.020...which is not super tight.

You are saying that a good bushing would be as smooth as, for example, rocker arms or the cam in the carrier (i.e. no movement at all but very smooth movement..

And what about the chamfer?

Cheers,

Mike

[cgarr]

I would get them rebuilt if you have any movement? they should push thru with some pressure but not loose, I see the ware limits on some of these parts and have no idea how they get away with it, I would not myself.. If the shop can not chamfer the bearing then find anothe shop, That has to be done to rocker bushings when you have to make them yourself!

[davidppp]

Hello, Mike.

Yes, good bushings do not rock on neewlightly oiled pins..IMHO..

Kind regards
David

[Mike Juzenas]

Thanks David!

My spec book gives the clearances at .020mm - 0.037. I will have the new bushings machined to 0.020 clearance from my pins (which are all very close to the same diameter and within spec) and check what they feel like oiled. BTW when I did the "wiggle test" that was with a dry bushing...I am sure that having oil in the hole makes a difference.

Cheers,

Mike

[bruboy]

Mike

I used to rebuild con rods and normally the small end bearings were machined to fit the pin as a light sliding fit without oil. You will however need the right porsche replacement bearing. As it is likely that it will be steel cased bearing with a bronze wearing face. All the original bearings I fitted had the oil hole relief already. If you shop says they cannot do this then it is most likeky that they are not fitting original bearings and will be machining some Phosphure bronze bar to fit???? I would ask you machine shop first and perhaps double check that the bushes are available from Porsche!


Your figures show the clearances right in the middle of the tolerance band so I would leave well alone if you are confident of the measurements.

If you do re- bush, reaming alone will not get the accuracy or maintain the center centre measurements betweeen the small and big end bearing. The bushes need to be bored first having set the correct the centre distances. We had a special con rod reconditoning machine for this with specially desigend fixturing. Make sure your local shop sets this dimension correctly or you will be in to changing deck heights again.

Although popular with the customers the reconditoning of the big end housing by shrinking the bore and resizing IMHO was a waste of time as very few rods in the thousands a did actually needed this work to be done. It also puts bending stresses on the cap bolts as the cap faces are machined slightly out square to shink the bore.

Probably more info than you needed!

[Mike Juzenas]

Originally posted by bruboy
Mike

I used to rebuild con rods and normally the small end bearings were machined to fit the pin as a light sliding fit without oil. You will however need the right porsche replacement bearing. As it is likely that it will be steel cased bearing with a bronze wearing face. All the original bearings I fitted had the oil hole relief already. If you shop says they cannot do this then it is most likeky that they are not fitting original bearings and will be machining some Phosphure bronze bar to fit???? I would ask you machine shop first and perhaps double check that the bushes are available from Porsche!


Your figures show the clearances right in the middle of the tolerance band so I would leave well alone if you are confident of the measurements.

If you do re- bush, reaming alone will not get the accuracy or maintain the center centre measurements betweeen the small and big end bearing. The bushes need to be bored first having set the correct the centre distances. We had a special con rod reconditoning machine for this with specially desigend fixturing. Make sure your local shop sets this dimension correctly or you will be in to changing deck heights again.

Although popular with the customers the reconditoning of the big end housing by shrinking the bore and resizing IMHO was a waste of time as very few rods in the thousands a did actually needed this work to be done. It also puts bending stresses on the cap bolts as the cap faces are machined slightly out square to shink the bore.

Probably more info than you needed!

Ok, what happened is I received my information from the guy at the front desk rather than the machinist. When I talked to him it was a different story and I had the following done:

- New Porsche small end bushings pressed in and reamed at the correct center distance. The pins have been polished and are a nice dry sliding fit. The oil holes were drilled and then reamed from the inside. They look like the originals. Now the hole is round not oval like the original bushing. On the old bushing I could slide the pin in and feel the difference in the wear between up-down and side-to-side. Now I can't.

- One the big end my rods were close but required a bit of work to be put back into round. I also changed the bolts to ARP bolts which affects the alignment. The shop did have to do some minor machining to get the bores back in round. It has been all double checked to make sure if the rods are torqued down with the bearing shell that I have about 2 thou clearance to the crank.

I will be checking my deck height anyways but they did not have to take much off...I will see what affect it has. In a way the pistons on the old bushings were going higher than now because of the wear!

Thanks for the comments this list is great!

Cheers,

Mike